Add multi-account financial overview

[alceops]

Summary

• Adds a financial_accounts data model and /accounts CRUD API for checking/savings/credit/cash/investment/loan accounts.
• Lets expenses optionally attach to account_id and adds /accounts/overview?month=YYYY-MM with per-account balances, assets, liabilities, net worth, and unassigned activity.
• Updates schema/docs and hardens Redis-dependent auth/cache paths so local/free-tier tests keep working when Redis is unavailable.

Closes #132

Tests

• PYTHONPATH=. ../../.venv/bin/pytest tests (24 passed)